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1Dr. L. Christofi Spring 2009
Lecture 4
Multiplexing and Spreading
Circuit Switching and Telephone Network
ACOE412
Data Communications
Spring 2009
2Dr. L. Christofi Spring 2009
0. Overview0. Overview
In this lecture we will cover the following topics:
6. Multiplexing and Spreading6.1 Multiplexing6.2 Spread spectrum6.3 Summary (part 6)
7. Circuit Switching and Telephone Network7.1 Circuit-switched networks7.2 Datagram networks7.3 Virtual circuit networks7.4 Structure of a switch7.5 Telephone network7.6 Dial-up modem7.7 Digital subscriber line7.8 Summary (part 7)
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3Dr. L. Christofi Spring 2009
Bandwidth utilization is the wise use of
available bandwidth to achieve
specific goals.
Efficiency can be achieved by multiplexing; privacy
and anti-jamming can be achieved by spreading.
Note
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6.1 MULTIPLEXING6.1 MULTIPLEXING
Whenever the bandwidth of a medium linking two devices Whenever the bandwidth of a medium linking two devices
is greater than the bandwidth needs of the devices, the link is greater than the bandwidth needs of the devices, the link
can be shared. Multiplexing is the set of techniques that can be shared. Multiplexing is the set of techniques that
allows the simultaneous transmission of multiple signals allows the simultaneous transmission of multiple signals
across a single data link. As data and telecommunications across a single data link. As data and telecommunications
use increases, so does traffic.use increases, so does traffic.
Frequency Division Multiplexing (FDM) Wavelength Division Multiplexing (WDM)
Synchronous Time Division Multiplexing (Sync TDM)
Statistical Time Division Multiplexing (Stat TDM)
Topics discussed in this section:Topics discussed in this section:
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Dividing a link into channels
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Categories of multiplexing
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Frequency Division Multiplexing (FDM)
FDM is an analog multiplexing technique that combines analog signals.
Note
• FDM can be used when the BW of a link is greater than the combined
BW of signals to be transmitted.
• Signals generated by each sending device modulate different carrier
frequencies, which are then combined into a single composite signal
• Guard bands are used to prevent signals from overlapping
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FDM multiplexing example
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FDM demultiplexing example
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Assume that a voice channel occupies a bandwidth of 4 kHz.
We need to combine three voice channels into a link with a bandwidth of 12 kHz, from 20 to 32 kHz. Show the
configuration, using the frequency domain. Assume there are
no guard bands.
Solution
We shift (modulate) each of the three voice channels to a different bandwidth, as shown in figure in next slide. We use
the 20-24 kHz bandwidth for the first channel, the 24-28 kHz
bandwidth for the second channel, and the 28-32 kHz
bandwidth for the third one. Then we combine them as shown
in the figure.
Example
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11Dr. L. Christofi Spring 2009
Example
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Five channels, each with a 100-kHz bandwidth, are to be multiplexed
together. What is the minimum bandwidth of the link if there is a need for a
guard band of 10 kHz between the channels to prevent interference?
Solution
For five channels, we need at least four guard bands. This means that the
required bandwidth is at least
5 × 100 + 4 × 10 = 540 kHz
as shown in the figure below.
Example
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Four data channels (digital), each transmitting at 1 Mbps, use a satellite
channel of 1 MHz. Design an appropriate configuration, using FDM.
Solution
The satellite channel is analog. We divide it into four channels, each
channel having a 250-kHz bandwidth. Each digital channel of 1 Mbps is
modulated such that each 4 bits is modulated to 1 Hz. One solution is 16-
QAM modulation. Figure below shows one possible configuration.
Example
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Analog hierarchy
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The Advanced Mobile Phone System (AMPS) uses two bands. The first
band of 824 to 849 MHz is used for sending, and 869 to 894 MHz is used
for receiving. Each user has a bandwidth of 30 kHz in each direction.
How many people can use their cellular phones simultaneously?
Solution
Each band is 25 MHz. If we divide 25 MHz by 30 kHz, we get 833.33. In
reality, the band is divided into 832 channels. Of these, 42 channels are
used for control, which means only 832-42=790 channels are available for
cellular phone users.
Example
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Wavelength Division Multiplexing (WDM)
• WDM is designed to use the high data rate capability of fiber optic
cable.
• Using a fiber-optic cable for one single line wastes the available
bandwidth. Multiplexing allows us to connect several lines into one.
• WDM is conceptually the same as FDM, except that the multiplexing
and demultiplexing involve optical signals
WDM is an analog multiplexing technique to combine optical signals.
Note
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Prisms in wavelength-division
multiplexing and demultiplexing
• Combining and splitting of light sources are easily handled by a prism
• A prism bends a beam of light based on the angle of incidence and
the frequency
• Using this technique, a MUX can be made to combine several
input beams of light, each containing a narrow band of frequencies,
into one beam of wider band of frequencies
• A DEMUX can be made to reverse the process
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Time Division Multiplexing (TDM)
TDM is a digital multiplexing technique for combining
several low-rate channels into one high-rate one.
Note
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Synchronous Time Division Multiplexing
In synchronous TDM, the data rate of the link is n times faster, and the unit duration is n
times shorter.
Note
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Figure in next slide shows synchronous TDM with a data stream for each
input and one data stream for the output. The unit of data is 1 bit. Find (a)
the input bit duration, (b) the output bit duration, (c) the output bit rate, and
(d) the output frame rate.
Solution
a. The input bit duration is the inverse of the bit rate: 1/1 Mbps = 1 µs.
b. The output bit duration is one-fourth of the input bit duration, or 0.25 µs.
c. The output bit rate is the inverse of the output bit duration or 1/(4µs) or
4 Mbps. This can also be deduced from the fact that the output rate is 4
times as fast as any input rate; so the output rate = 4 × 1 Mbps = 4
Mbps.
d. The frame rate is always the same as any input rate. So the frame rate is
1,000,000 frames per second. Because we are sending 4 bits in each
frame, we can verify the result of the previous question by multiplying
the frame rate by the number of bits per frame.
Example
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Example
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Four 1-kbps connections are multiplexed together. A unit is 1 bit. Find (a)
the duration of 1 bit before multiplexing, (b) the transmission rate of the link,
(c) the duration of a time slot, and (d) the duration of a frame.
Solution
a. The duration of 1 bit before multiplexing is 1 / 1 kbps, or 0.001 s (1 ms).
b. The rate of the link is 4 times the rate of a connection, or 4 kbps.
c. The duration of each time slot is one-fourth of the duration of each bit
before multiplexing, or 1/4 ms or 250 µs. Note that we can also
calculate this from the data rate of the link, 4 kbps. The bit duration is
the inverse of the data rate, or 1/4 kbps or 250 µs.
d. The duration of a frame is always the same as the duration of a unit
before multiplexing, or 1 ms. We can also calculate this in another way.
Each frame in this case has four time slots. So the duration of a frame
is 4 times 250 µs, or 1 ms.
Example
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Interleaving
• TDM can be visualized as two fast rotating switches, one on the MUX
side and the other on the DEMUX side. The switches are synchronized
and rotate at the same speed but in opposite directions. On the MUX
side, as the switch opens in front of a connection, that connection has
the opportunity to send a unit onto the path. This process is called
inteleaving.
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Four channels are multiplexed using TDM. If each channel sends 100
bytes /s and we multiplex 1 byte per channel, show the frame traveling on
the link, the size of the frame, the duration of a frame, the frame rate, and
the bit rate for the link.
Solution
The multiplexer is shown below. Each frame carries 1 byte from each
channel; the size of each frame, therefore, is 4 bytes, or 32 bits. Because
each channel is sending 100 bytes/s and a frame carries 1 byte from each
channel, the frame rate must be 100 frames per second. The bit rate is 100
× 32, or 3200 bps.
Example
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A multiplexer combines four 100-kbps channels using a time slot of 2 bits.
Show the output with four arbitrary inputs. What is the frame rate? What is
the frame duration? What is the bit rate? What is the bit duration?
Solution
Figure below shows the output for four arbitrary inputs. The link carries
50,000 frames per second. The frame duration is therefore 1/50,000 s or 20
µs. The frame rate is 50,000 frames per second, and each frame carries 8
bits; the bit rate is 50,000 × 8 = 400,000 bits or 400 kbps. The bit duration is
1/400,000 s, or 2.5 µs.
Example
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Digital hierarchy
Telephone companies implement TDM through a hierarchy of digital
signals, called Digital Signal (DS) Service.
The following figure shows the data rates supported by each level:
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DS and T line rates
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T-1 line for multiplexing telephone lines
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T-1 frame structure
DS-1 requires 8 kbps overhead due to the synchronization bit:
T1 line = 24 slots x 8 bits + 1 bit for synchronization = 193 bits x 8kbps
= 1.544Mbps
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E line rates
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TDM slot comparison
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6.2 SPREAD SPECTRUM6.2 SPREAD SPECTRUM
In spread spectrum (SS), we combine signals from different In spread spectrum (SS), we combine signals from different
sources to fit into a larger bandwidth, but our goals are to sources to fit into a larger bandwidth, but our goals are to
prevent eavesdropping and jamming. To achieve these prevent eavesdropping and jamming. To achieve these
goals, spread spectrum techniques add redundancy.goals, spread spectrum techniques add redundancy.
Frequency Hopping Spread Spectrum (FHSS)
Direct Sequence Spread Spectrum Synchronous (DSSS)
Topics discussed in this section:Topics discussed in this section:
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33Dr. L. Christofi Spring 2009
Spread spectrum
• Input is fed into a channel encoder that produces an analog signal
with a relatively narrow BW around a center frequency.
• This signal is further modulated using a sequence of digits known as a
spreading code or spreading sequence.
• The effect of this modulation is to increase significantly the BW
(spread the spectrum) of the signal to be transmitted.
• On the receiving end, the same digit sequence is used to demodulate
the spread spectrum signal.
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Frequency hopping spread spectrum (FHSS)
With FHSS the signal is broadcast over a random series of
radio frequencies, hopping from frequency to frequency at fixed intervals.
A receiver, hopping between frequencies in
synchronization with the transmitter picks up the message.
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35Dr. L. Christofi Spring 2009
FHSS system
For transmission, binary data are fed into a modulator using FSK or
PSK. The resulting signal is entered on a base frequency. A
pseudorandom code generator serves as an index into a table of
frequencies (spreading code). Each k bits of the spreading sequence
specifies one of the 2k carrier frequencies. At each k-bit interval a new
carrier frequency is selected.
This frequency is then
modulated to produce a
new signal with the same
shape but centered on the
selected carrier frequency.
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Frequency selection in FHSS
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FHSS cycles
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Bandwidth sharing
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Direct Sequence Spread Spectrum (DSSS)
With DSSS each bit in the original signal is represented by multiple bits
in the transmitted signal using a spreading code.
The spreading code spreads the signal across a wider frequency band
in direct proportion to the number of bits used.
eg. a 10-bit spreading spreading code spreads the signal
across a frequency band that is 10 times greater than a 1-bit
spreading code.
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DSSS example
In this example, the information is combined with trhe spreading code
using an XOR operation
i.e. 0 + 0 = 0 0 + 1 = 1 1 + 0 = 1 1 + 1 = 0
0 1 0 0 1 0 0 0 1 1 1 1 0 1 1 0 1 1 1 0 0 0 0 1 0 0 1 0 0 0 1 1 1
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41Dr. L. Christofi Spring 2009
6.3 SUMMARY (part 6)
• Multiplexing is the simultaneous transmission of multiple signals across a single data link.
• Frequency-division multiplexing (FDM) and wave-division multiplexing (WDM) are techniques for analog signals, while time-division multiplexing (TDM) is for digital signals.
• In FDM, each signal modulates a different carrier frequency. The modulated carriers are combined to form a new signal that is then sent across the link.
• In FDM, multiplexers modulate and combine signals while demultiplexers decompose and demodulate.
• In FDM, guard bands keep the modulated signals from overlapping and interfering with one another.
• Telephone companies use FDM to combine voice channels into successively larger groups for more efficient transmission.
• Wave-division multiplexing is similar in concept to FDM. The signals being multiplexed, however, are light waves.
• In TDM, digital signals from n devices are interleaved with one another, forming a frame of data (bits, bytes, or any other data unit).
• Framing bits allow the TDM multiplexer to synchronize properly.
• Digital signal (DS) is a hierarchy of TDM signals.
• T lines (T-1 to T-4) are the implementation of DS services. A T-1 line consists of 24 voice channels.
• T lines are used in North America. The European standard defines a variation called E lines.
• Inverse multiplexing splits a data stream from one high-speed line onto multiple lower-speed lines.
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Switched network
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Taxonomy of switched networks
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7.1 CIRCUIT7.1 CIRCUIT--SWITCHED NETWORKSSWITCHED NETWORKS
A circuitA circuit--switched network consists of a set of switches switched network consists of a set of switches
connected by physical links. A connection between two connected by physical links. A connection between two
stations is a dedicated path made of one or more links. stations is a dedicated path made of one or more links.
However, each connection uses only one dedicated However, each connection uses only one dedicated
channel on each link. Each link is normally divided into n channel on each link. Each link is normally divided into n
channels by using FDM or TDM.channels by using FDM or TDM.
Three Phases
Efficiency
Delay
Circuit-Switched Technology in Telephone Networks
Topics discussed in this section:Topics discussed in this section:
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A circuit-switched network is made of a set of
switches connected by physical links, in which each
link is divided into n channels.
Note
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A simple circuit-switched network
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In circuit switching, the resources need to be
reserved during the setup phase;the resources remain dedicated for the entire duration
of data transfer until the teardown phase.
Note
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As an example, let us use a circuit-switched network to connect eight
telephones in a small area. Communication is through 4-kHz voice
channels. We assume that each link uses FDM to connect a maximum of
two voice channels. The bandwidth of each link is then 8 kHz. Figure shows
the situation. Telephone 1 is connected to telephone 7; 2 to 5; 3 to 8; and 4
to 6. Of course the situation may change when new connections are made.
The switch controls the connections.
Example
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49Dr. L. Christofi Spring 2009
As another example, consider a circuit-switched network that connects
computers in two remote offices of a private company. The offices are
connected using a T-1 line leased from a communication service provider.
There are two 4 × 8 (4 inputs and 8 outputs) switches in this network. For
each switch, four output ports are folded into the input ports to allow
communication between computers in the same office. Four other output
ports allow communication between the two offices. Figure 8.5 shows the
situation.
Example
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Delay in a circuit-switched network
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Switching at the physical layer in the traditional
telephone network uses
the circuit-switching approach.
Note
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7.2 DATAGRAM NETWORKS7.2 DATAGRAM NETWORKS
In data communications, we need to send messages from In data communications, we need to send messages from
one end system to another. If the message is going to pass one end system to another. If the message is going to pass
through a packetthrough a packet--switched network, it needs to be divided switched network, it needs to be divided
into packets of fixed or variable size. The size of the packet into packets of fixed or variable size. The size of the packet
is determined by the network and the governing protocol.is determined by the network and the governing protocol.
Routing Table
EfficiencyDelay
Datagram Networks in the Internet
Topics discussed in this section:Topics discussed in this section:
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In a packet-switched network, there
is no resource reservation;
resources are allocated on demand.
Note
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A datagram network with four routers
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Routing table in a datagram network
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A switch in a datagram network uses a routing table that is based on the destination address.
Note
The destination address in the header of a packet in a datagram network
remains the same during the entire journey of the packet.
Note
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Delay in a datagram network
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Switching in the Internet is done by using the datagram approach
to packet switching at the network layer.
Note
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7.3 VIRTUAL7.3 VIRTUAL--CIRCUIT NETWORKSCIRCUIT NETWORKS
A virtualA virtual--circuit network is a cross between a circuitcircuit network is a cross between a circuit--
switched network and a datagram network. It has some switched network and a datagram network. It has some
characteristics of both.characteristics of both.
Addressing
Three Phases
Efficiency
DelayCircuit-Switched Technology in WANs
Topics discussed in this section:Topics discussed in this section:
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Virtual-circuit network
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Virtual-circuit identifier
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Switch and tables in a virtual-circuit network
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Source-to-destination data transfer in a
virtual-circuit network
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Setup request in a virtual-circuit network
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Setup acknowledgment in a virtual-
circuit network
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In virtual-circuit switching, all packets belonging to the same source and
destination travel the same path;but the packets may arrive at the destination with
different delays
if resource allocation is on demand.
Note
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Delay in a virtual-circuit network
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Switching at the data link layer in a switched WAN is normally
implemented by using virtual-circuit techniques.
Note
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7.4 STRUCTURE OF A SWITCH7.4 STRUCTURE OF A SWITCH
We use switches in circuitWe use switches in circuit--switched and packetswitched and packet--switched switched
networks. In this section, we discuss the structures of the networks. In this section, we discuss the structures of the
switches used in each type of network.switches used in each type of network.
Structure of Circuit Switches
Structure of Packet Switches
Topics discussed in this section:Topics discussed in this section:
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Time-slot interchange
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Time-space-time switch
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Packet switch components
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Input port
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Output port
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7.5 TELEPHONE NETWORK7.5 TELEPHONE NETWORK
Telephone networks use circuit switching. The telephone Telephone networks use circuit switching. The telephone
network had its beginnings in the late 1800s. The entire network had its beginnings in the late 1800s. The entire
network, which is referred to as the network, which is referred to as the plain old telephone plain old telephone
systemsystem ((POTSPOTS), was originally an analog system using ), was originally an analog system using
analog signals to transmit voice.analog signals to transmit voice.
Major Components
LATAs
Signaling
Services Provided by Telephone Networks
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A telephone system
Endoffices
Local loop
Trunk
Tandemoffices Regional offices
Trunk
• • •
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Intra-LATA services are provided by Local Exchange
Carriers.
Since 2003, in Cyprus there are two
types of LECs: incumbent local exchange carriers and
competitive local exchange carriers.
Note
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Switching offices
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Pointof presences (POPs)
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The tasks of data transfer and signaling are separated in modern telephone networks: data transfer is done
by one network, signaling by another.
Note
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Data transfer and signaling networks
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Layers in SS7
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7.6 DIAL7.6 DIAL--UP MODEMSUP MODEMS
Traditional telephone lines can carry frequencies between Traditional telephone lines can carry frequencies between
300 and 3300 Hz, giving them a bandwidth of 3000 Hz. All 300 and 3300 Hz, giving them a bandwidth of 3000 Hz. All
this range is used for transmitting voice, where a great deal this range is used for transmitting voice, where a great deal
of interference and distortion can be accepted without loss of interference and distortion can be accepted without loss
of intelligibility.of intelligibility.
Modem Standards
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Telephone line bandwidth
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Modulation/demodulation
Modem
stands for modulator/demodulator.
Note
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The V.32 and V.32bis constellation and
bandwidth
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Uploading and downloading in 56K modems
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7.7 DIGITAL SUBSCRIBER LINE7.7 DIGITAL SUBSCRIBER LINE
After traditional modems reached their peak data rate, After traditional modems reached their peak data rate,
telephone companies developed another technology, DSL, telephone companies developed another technology, DSL,
to provide higherto provide higher--speed access to the Internet. speed access to the Internet. Digital Digital
subscriber linesubscriber line ((DSLDSL) technology is one of the most ) technology is one of the most
promising for supporting highpromising for supporting high--speed digital communication speed digital communication
over the existing local loops. over the existing local loops.
ADSL
ADSL LiteHDSL
SDSL
VDSL
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ADSL is an asymmetric communication technology
designed for residential users; it is not suitable for
businesses who want high upload speeds.
Note
The existing local loops can handle bandwidths up to about 4-5 MHz.
Note
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ADSL is an adaptive technology. The system uses a data rate
based on the condition of the local loop line.
Note
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Discrete multitone technique
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Bandwidth division in ADSL
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ADSLmodem
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DSLAM
Digital Subscriber Line Access Multiplexer
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Summary of DSL technologies
(km)
4
6
4
4
1-2.5
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7.8 SUMMARY (part 7)
• Switching is a method in which communication devices are connected to one another efficiently.
• A switch is intermediary hardware or software that links devices together temporarily. • There are three fundamental switching methods: circuit switching, packet switching, and
message switching.
• In circuit switching, a direct physical connection between two devices is created by space-division switches, time-division switches, or both.
• In a space-division switch, the path from one device to another is spatially separate from other paths.
• A crossbar is the most common space-division switch. It connects n inputs to m outputs via n × m crosspoints.
• Multistage switches can reduce the number of crosspoints needed, but blocking may result. • Blocking occurs when not every input has its own unique path to every output.
• In a time-division switch, the inputs are divided in time, using TDM. A control unit sends the input to the correct output device.
• The time-slot interchange and the TDM bus are two types of time-division switches. • Space- and time-division switches may be combined.
• A telephone network is an example of a circuit-switched network.
• A telephone system has three major components: local loops, trunks, and switching offices.
• Telephone companies provide digital services such as switched/56 services and digital data services.
• A home computer can access the Internet through the existing telephone system or through a
cable TV system.
• DSL supports high-speed digital communications over the existing telephone local loops.
• ADSL technology allows customers a bit rate of up to 1 Mbps in the upstream direction and up to
8 Mbps in the downstream direction.
• ADSL uses a modulation technique called DMT which combines QAM and FDM.
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References
• W. Stalling, Local and Metropolitan Area Networks, 6th edition, Prentice Hall, 2000
• F. Halsall, Data Communications, Computer Networks and Open Systems, 4th edition, Addison Wesley, 1995
• B.A. Forouzan, Data Communications and Networking, 4th edition, McGraw-Hill, 2007
• W. Stallings, Data and Computer Communications, 7th edition, Prentice Hall, 2004